While specific cellular mutational mechanistic pathways are often most conveniently studied in vitro, differences in tissue response, in chemical deposition or in the response of different cell types to mutagen/carcinogen exposure can only be defined in vivo. Our goal is to take advantage of the techniques of molecular biology to devise strategies and models using transgenic rodent models for the quantitative determination of in vivo mutations. We continue to define the experimental and statistical methods useful for analyzing mutations in vivo have continued. Areas of assay variability have been carefully defined and statistical approaches for the analysis of data have been developed. In addition, we are using the Big Blue system to assess the effects of cellular proliferation on the yield of mutations either in the presence or absence of DNA damage. The in vivo mutagenic activity of the carcinogen/noncarcinogen pair, 2,4- or 2,6-diaminotoluene correlates with the carcinogenic potential of these isomers suggesting that cellular proliferation induced by 2,4-DAT appears to be a necessary prerequisite for generating a mutational response in Big Blue mice. Finally, we have further studied the effects of cellular proliferation by subjecting the mice to partial hepatectomy following treatment with benzo[a]pyrene (B[a]P). The increase in MF at sacrifice was significantly different in the treated mice compared to the controls. However, the MF at sacrifice was not significantly different from that at the time of hepatectomy, suggesting that MF is not altered to any extent in the replicating cells if the MF is evaluated before the liver has completely regenerated. These data underscore the importance of timing when evaluating the effects of cell proliferation on mutation fixation in vivo.